The present invention relates to a multi-channel decoding scheme, and more particularly to a multi-channel decoding system capable of reducing noise when switching between different channel modes, and method thereof.
Generally speaking, when a multi-channel decoding system switches from a multi-channel mode to a single-channel mode or from the single-channel mode to the multi-channel mode, users often hear a “pop” noise from their earphones or speakers. This is primarily because the output DC voltage levels of a channel output signal in the multi-channel and single-channel modes, which is generated from the multi-channel decoding system, are different. Another reason may result from the Glitch phenomenon during switching between the multi-channel and single-channel modes. Taking a stereo decoding system as an example, the pop noise is usually heard by human when the stereo decoding system switches between stereo and mono modes.
Please refer to FIG. 1 in conjunction with FIG. 2. FIG. 1 is a diagram of a related art stereo decoding system 100, and FIG. 2 is a timing diagram illustrating a clock signal SCLK having a frequency 38 KHz, a mode switching signal SMODE, and a clock signal SCLK′. As shown in FIG. 1, the stereo decoding system 100 comprises a decoding circuit 105 and a clock generating circuit 110, where the decoding circuit 105 further includes a mixer 115, voltage-to-current (V/I) converters 120a and 120b, a separation module 125, operational amplifiers OP1 and OP2, resistor units 130a and 130b, and low pass filters (LPFs) 135a and 135b. The decoding circuit 105 is utilized for receiving an input audio signal Sin to generate a left channel output signal LOUT and a right channel output signal ROUT according to the clock signal SCLK′. When the mode switching signal SMODE is at a high logic level, it indicates that the stereo decoding system 100 is in the stereo mode; when the mode switching signal SMODE is at a low logic level, this indicates that the stereo decoding system 100 is in the mono mode.
Ideally, the DC voltage level of the channel output signal LOUT or ROUT should be identical whether the stereo decoding system 100 is in the stereo mode or the mono mode. Practically, however, an equivalent offset voltage source Vos exists within the decoding circuit 105 and the DC voltage level of the channel output signal LOUT/ROUT in the stereo and mono modes are different due to the offset voltage source Vos. For example, in stereo mode, a voltage of the offset voltage source Vos is equal to V1; in mono mode, the voltage of the offset voltage source Vos will become V2. Consequently, the DC voltage level of the channel output signal LOUT/ROUT is changed while the stereo decoding system 100 switches from mono mode to stereo mode and the above-mentioned pop noise is thus introduced.